In EDM, a tool electrode in the form of a three-dimensionally shaped solid body (i.e. sinking-type EDM) or a continuous wire or like elongated body (wire-cut EDM) is juxtaposed with a workpiece across a machining gap filled with a machining fluid or liquid dielectric (e.g. kerosene, transformer oil, distilled water or weakly conductive water). Electric energy in the form of discrete electrical pulses are supplied across the fluid-filled machining gap to effect a succession of electrical discharges between the tool electrode and the workpiece to remove material from the latter. As material removal proceeds, the tool electrode is advanced relatively toward the workpiece by servofeed means adapted to maintain the machining gap spacing substantially constant thereby to allowing material-removal discharges to be successively created. For example, in wire-cut (WC) EDM, also called traveling-wire (TW) EDM, the workpiece may be displaced along a predetermined cutting path in a plane transverse to the axis of the continuously axially moving traveling-wire electrode under numerical control (NC). In sinking-type EDM, the tool electrode may be advanced in the direction of its axis or along the Z-axis and additional relative movements may be imparted to the tool electrode and the workpiece in an X-Y plane orthogonal to the Z-axis to generate a cavity in the workpiece.
Parameters of individual and successive electrical discharges, i.e. pulse on-time (.tau.on), peak current (Ip) and off-time (.tau.off), are determinative of machining results, e.g. removal rate, surface roughness and relative electrode wear and, therefore, are individually or in combination particularly adjusted to establish a particular machining condition suitable for achieving desired machining results.
The contamination of the machining gap region with chips, tar and gases produced by machining discharges may be removed by continuously or intermittently flushing the gap with the fresh machining fluid and/or intermittently or cyclically retracting the tool electrode away from the workpiece to allow the fresh machining medium to be pumped into the machining gap and the machining contaminants to be carried away from the latter.
In order to increase the removal rate, it is also desirable to operate the electrode-feed servocontrol system so as to minimize production of non-striking pulses or pulses which do not cause discharge. The gap spacing can accordingly be reduced but this can also facilitate production of an continuous arc or result in difficulty in gap flushing or decontamination. The attempt to facilitate machining discharges tends to drop the removal rate while the setting of the servo system so as to widen the threshold gap spacing leads to an increased frequency of non-striking pulses.
As noted above, the critical parameters involved in EDM include, apart from the electrode and workpiece materials and dielectric compositions, machining pulse parameters, dielectric supply, gap spacing and gap decontamination. In addition, especially in WC-EDM or TW-EDM, the wire axial transport velocity, the dielectric resistivity and temperature, and the workpiece thickness must be controlled or, for control purposes, be strictly taken into consideration. Control can respond to monitoring the status or machining condition in the EDM gap in which machining is in progress. Because of instantaneous and constant change, however, that the gap must experience in the EDM process, it has long been recognized in the art that it is extremely difficult to perform control operations for these parameters in combination in a manner to yield optimal results for varying machining requirements. The difficulty arises from the fact that the EDM process involves extremely complex phenomena and the parameters which govern the process are mutually interrelated in highly complicated ways over extensive ranges.
It should also be noted that with ever increasing resort to industry in recent years there have been imposed increasingly diverse complex operational requirements on an EDM equipment. To meet these requirements, therefore, it has become mandatory that a machine be equipped with highly complicated and excessively costly and massive control circuits.